The World Health Organization said in a written statement Sunday that doctors still have a lot of questions and concerns about the virus.

"Of most concern, however, is the fact that the different clusters seen in multiple countries increasingly support the hypothesis that when there is close contact this novel coronavirus can transmit from person to person."

Some common colds are also caused by a coronavirus. And like a cold, NCoV infection attacks the respiratory system, the Centers for Disease Control and Prevention have said.

But symptoms are severe and can lead to pneumonia and even kidney failure.

"Once it gets you, it's a very serious infection," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center in Nashville, Tennessee.

NCoV has also been compared to a related coronavirus, the one that causes SARS, or severe acute respiratory syndrome, which appears to have been contained in 2004, according to the CDC.

But the WHO said: "This new virus is not the SARS virus. They are distinct from each other. However, the fact that they are related has added to the world's concern."

The organization said most of the people who have been reported to have an infection are older men with other medical conditions. Precise data are not available on the total number of people who have been infected because it is difficult to ascertain how many get a mild form of NCoV.

The WHO said it is an "important and major challenge" to solve the riddle of what causes the infection, where it originates and how widespread it is.

Infectious disease specialists believe the virus is very difficult to catch.

The two infected men are the first confirmed cases in France. The first patient had recently returned from the United Arab Emirates, the French Health Ministry said.

Identifying the source of that man's ailment took time, the ministry said. France's Pasteur Institute confirmed the case Wednesday. The Health Ministry identified 124 people who had contact with the man.